The invention relates to the field of thin film optical devices. More particularly, the present invention relates to transparent electroactive cavity thin film optical filters.
Highly accurate optical interference filters can be manufactured using thin film deposition processes. These devices are typically built by depositing alternating layers of transparent materials where one layer possesses a much larger refractive index relative to the other layer. Theoretically, the proper choice of composition, thickness and quantity of layers could result in a device with any desired transmission spectrum. Among the simplest devices is the single cavity bandpass filter, that is the thin-film form of an etalon. This device consists of three sets of layers. The first stack is a dielectric mirror. This is followed by a thicker layer that forms the spacer. This is followed by another stack that forms a dielectric mirror. The mirror stacks are typically fabricated by depositing alternating transparent materials that have an optical thickness that is one quarter of the optical wavelength of light. To achieve the theoretical optical performance, each layer must possess a precise and specific physical thickness and refractive index. Any nonuniformity in the deposition of the layers can affect the spectral placement and transmission or reflection characteristics of the device. A design that requires very tight manufacturing tolerances over large substrate areas could result in the costly rejection of many devices. Given these manufacturing limits, it would be desirable to analyze the devices after construction and alter the devices that do not meet a predetermined optical transmission or reflection specification by some electrical or mechanical means. For example, if the peak transmission wavelength of a manufactured optical bandpass cavity filter was slightly out of tolerance, it would be desirable to have a means for shifting the peak back to the desired spectral location. It is also desirable that the optical filters have precise rejection bands and passbands that are electrically or mechanically selectable.
Mechanical methods of achieving a variable transmission spectrum device are well known. This includes changing a prism or grating angle, or altering the optical spacing between mirrors of an etalon. To overcome the performance, size and cost disadvantages of using mechanical schemes, many have conceived of electrical methods for varying a transmission spectrum. For example, U.S. Pat. No. 5,150,236 issued Sep. 22, 1999 discloses a tunable liquid crystal etalon filter. The liquid crystal fills the space between dielectric mirrors. Electrodes on the mirrors are used to apply an electric field, which changes the orientation of the liquid crystal that changes the optical length for tuning. The change in the optical length corresponds to a change in the location of the passband. In addition, U.S. Pat. No. 5,103,340 issued Apr. 07, 1992 discloses piezoelectric elements placed outside the optical path, which are used to change the spacing between cascaded cavity filters. Furthermore, U.S. Pat. No. 5,799,231 discloses a variable index distributed mirror. This is a dielectric mirror with half of the layers having a variable refractive index that is matched to other layers. Changing the applied field increases the index difference that increases the reflectance. The mathematics that describe the transmission characteristics of multilayer films composed of electro-optic and dielectric materials are well known.
Another electrically actuated thin film optical filter uses a series of crossed polarizers and liquid crystalline layers that allow electrical controls to vary the amount of polarization rotation in the liquid by applying an electric field in such a way that some wavelengths are selectively transmitted. However, these electrically actuated thin film optical filters have the characteristic that the light must be polarized and that the frequencies of light not passed are absorbed, not reflected. Another electrically actuated thin film optical device is the tunable liquid crystal etalon optical filter. The tunable liquid crystal etalon optical filter uses a liquid crystal between two dielectric mirrors. U.S. Pat. No. 5,710,655 issued Jan. 20, 1998 discloses a cavity thickness compensated etalon filter.
The common cavity filter, such as the etalon optical filter, is an optical filter with one or more spacer layers that are deposited in the stack and that define the wavelength of the rejection and pass bands. The optical thickness of the film defines the placement of the passband. In the tunable liquid crystal etalon optical filter, an electric field is applied to the liquid crystal that changes the optical length between the two mirrors so as to change the passband of the etalon. Still another tunable optical filter device tunes the passband by using piezoelectric elements to mechanically change the physical spacing between mirrors of an etalon filter. Bulk dielectrics are made by subtractive methods like polishing from a larger piece; whereas thin film layer are made by additive methods like vapor or liquid phase deposition. A bulk optical dielectric, for example, greater than ten microns, disposed between metal or dielectric mirrors suffer from excessive manufacturing tolerances and costs, the bulk material providing unpredictable, imprecise, irregular, or otherwise undesirable passbands. These electrical and mechanical optical filters disadvantageously do not provide precise rejection bands and passbands that are repeatably manufactured. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide a tunable optical interference filter.
Another object of the invention is to provide a tunable optical filter with stable rejection and passbands having electrooptic medium.
Yet another object of the invention is to provide tunable optical filter with stable rejection and passbands manufactured using conventional thin film deposition processes.
Still another object of the invention is to provide tunable optical filter with stable rejection and passbands that is polarization insensitive.
A further object of the invention is to provide a tunable optical filter with stable rejection with stacked cavities made of transparent electroactive material forming an optical cavity.
Yet a further object of the invention is to provide a tunable optical filter with stable rejection and passbands using a series of optical cavities made of transparent electroactive materials forming an optical device having a refractive index that changes the resonant wavelength with an applied voltage to provide a tunable passband shift.
The invention is directed to a thin film tunable optical filter that provides highly accurate passbands yet manufactured using conventional manufacturing processes. After fabrication, the optical properties of the filter are stable with a degree of tunability. A tuning voltage is applied to an optical interference stack that shifts spectral features such as the transmission passband by a predetermined amount. The optical filter includes the use of a solid state electrooptic medium manufactured in a single chamber using conventional thin film deposition processes that grow transmissive material in layers of up to ten microns. Passband tuning occurs without moving any of the elements so that the optical filter is robust and vacuum compatible. The optical filter is polarization insensitive and reflects energy not transmitted with many applications including multiplexing and demultiplexing of optical signals.
The optical filter uses a tunable spacer. One or more spacer transparent electroactive material spacer layers are deposited in the stack to define the resonant wavelength of the device. The use of multiple electrooptic spacer layers as coupled cavities serve to widen and narrow the optical passband. The use of materials compatible with the single chamber deposition process provides a manufacturable monolithic device. The optical thickness of the film spacer layers defines the spectral placement of the passband. The solid state electrooptic material enables changes in the optical thickness of the spacer layers to shift the passband wavelength. The use of multiple spacers function to electrically alter multiple coupled cavities and thus shift the passbands. The optical filter can be controlled to electrically narrow or broaden the spectral width of the passband. When the passbands overlap, one spacer layer can define the long wavelength band edge and another spacer layer can define the short wavelength edge. The refractive index changes in response to changes in an applied tuning voltage producing changes in the resonant wavelength to shift the passband.
The electroactive cavity is disposed between a transparent conductor like indium tin oxide or very thin metal to deliver the charge. The conductive layer can be patterned so that the filter is divided into pixels that are individually addressable over the surface of the filter. Patterning the transparent conductor provides an addressable array of tunable filters. Pixelization is useful in improving uniformity over a large optical area. When applied in conjunction with a detector array, pixelization allows proximal detector elements to be illuminated with different spectra of light.
Parallel production of monolithic optical filters significantly reduces cost. The layers are deposited as a repeatable multilayer stack on a large wafer that can then be diced into very small pieces. Each piece can be tested for wavelength placement and band shape. With repeatable fabrication processes identical filters can be used for multiple channel application. Each filter may have a slightly different voltage due to small manufacturing tolerances that can be compensated by changing the applied field. A set of filters can be manufactured as a single stage demultiplexer that could be tuned to any channel desired to eliminate the need for the whole filter set. The compensating applied voltage can be programmed to compensate for temperature dependence as well. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.